1. Field of the Invention:
This invention relates to an adhesive for electroless plating used for a circuit board and a method of preparation of a circuit board using this adhesive and, more particularly, to an adhesive for electroless plating, which is excellent in heat resistance, electric characteristics and adhesiveness between a substrate and an electroless plating film, and a method of preparation of a circuit board using this adhesive.
2. Description of the Prior Art:
Recently, miniaturization and speeding up of electronic device have been developed with the progress of electronic industries. Accordingly, in the circuit board wherein a printed circuit board and a LSI are practically mounted, it has been required a high density and a high reliability due to fine pattern.
Heretofore, as a method for forming a conductor circuit on a printed circuit board, an etched foil method for forming a conductor circuit by photoetching after laminating copper foil on a substrate is widely used. According to this method, a conductor circuit excellent in the adhesiveness to the substrate can be formed. However, this method has such defect that a fine pattern having a high accuracy with etching can not be obtained due to a large thickness of the copper foil. Further, the preparing steps are so complicated that good efficiency can not be obtained.
Heretofore, as substrate materials used for said circuit board, ceramic materials having good reliability, stability of sizing, heat diffusion or the like are widely used. As a method for forming a conductor circuit on a circuit board made from said ceramic material, it is generally applied a method in which a thick film conductor paste such as paste of silver palladium series is printed by screen printing.
However, the conductor formed by said thick film paste is high in sheet resistance, and great in transmission loss of high frequency. Further, it is within the limits of 0.2 mm in width in fine line characteristics practically and is also inferior in soldering and bonding. Furthermore, it is necessary to pay attention to silver migration.
As a method for forming such conductor which is possible to dissolve the above defect, a method for forming a conductor on a substrate by electroless plating has been recently proposed. However, the conductor formed by electroless plating is inferior in adhesiveness to substrate materials. Accordingly, various researches for increasing such adhesiveness has been studied.
For instance, following methods described in the collection of previous copy of lecture in the 11th ISHM JAPAN technical lecture meeting, pages 9 to 17, distributed on Apr. 11, 1985, are adopted:
A method of electroless plating in which a surface of ceramic substrate is previously chemically etched to be roughed, thereby displaying anchor effect, and
An additive method in which an adhesive containing a synthetic rubber of diene series is coated on a surface of substrate to form an adhesive layer and electroless plating is carried out to form a conductor after roughing the surface of the adhesive layer.
However, these methods described in the collection of previous copy of lecture are considerably difficult to rough the whole surface of the ceramic substrate by uniform etching and adhesive strength of the conductor due to the electroless plating is apt to disperse, thereby being scarce of reliability. Further, since the adhesive generally used in the latter method contains the synthetic rubber, the latter method has such defects as to cause a large decrease of adhesive strength at high temperature and a blister of electroless plating film at soldering due to low heat resistance and insufficient electric characteristics such as surface resistance. Accordingly, the use of such adhesive is considerably restricted.
An invention regarding "resin composition for printed circuit board" is disclosed in the patent laid open No. Sho 53 (1978)-140344 Gazette, the summary of the invention as described in the claim is as follows;
"A resin composition for printed circuit board used for preparing a printed circuit board characterized in that said resin composition consists of a component (A) capable of being soluble by etching to form a continuous phase and a component (B) of a thermosetting resin, which is not soluble in phase of the component (A), and forms spherical particles having 0.5 to 15 .mu.m in diameter without being etched to disperse in the continuous phase of the component (A), the component (B) consists of a resin having 20 to 85 volume % to the sum of the components (A) and (B), and is coated on a substrate surface to be formed at least a part of a conductor contacting directly with a substrate by electroless plating". According to the invention described in said gazette, since the adhesive layer obtained by etching and roughing of the resin composition causes an uneveness having about 20 .mu.m in depth as described in page 3 on the left-hand column, the conductor formed on this adhesive layer is difficult to obtain fine patterns and insulation between the patterns is liable to be degraded. Further, it is not preferable to attach parts to such conductor.
As described the above, heretofore it is not well-known an adhesive, which possesses a heat resistance, electric characteristics, and a adhesiveness between substrate and electroless plating film and is possible to practise the electroless plating comparatively easily, and a method of preparation of a circuit board using this adhesive.